Most laminated organic electrophotographic photoreceptors currently produced have a charge generating layer laminated beneath a charge transporting layer. Since electrophotographic photoreceptors of this type use positive hole transport systems, they require negative charging. Accordingly, various defects such as nonuniformity in the charging of the corotoron and high ozone concentration formation are caused on the electrophotographic photoreceptor by the negative charge. Previous attempts to resolve these defects were aimed at constructing a photoreceptor wherein a charge generating layer was laminated above a charge transporting layer and used with a positive charge while a low resistance surface protective layer was laminated as the top layer in order to improve mechanical strength and quality, as described in, e.g., U.S. Pat. No. 4,409,309.
Electrophotographic photoreceptors of this type of construction, however, have various shortcomings and defects. For example, they fail to hold sufficient charge and thus produce unsatisfactory low contrast levels and low potentials. When they can hold a sufficient charge, however, light decay is insufficient, residual potential is too high, charge potential and residual potential fluctuate due to variations in temperature and humidity, charge potential and residual potential fluctuate under repeated use, and the decrease in sensitivity is greater than before the surface protective layer was added.
The use of symmetric squarylium acid derivatives as materials for the charge generating layer has been proposed for use in electrophotographic photoreceptors to avoid such shortcomings and defects, e.g., in U.S. Pat. Nos. 4,353,971, 4,521,621, 4,523,035, 4,524,218 and 4,752,650. However, squarylium acid derivatives comprising symmetric structures have certain defects and drawbacks. For example, they account for poor dispersibility, nonuniform coating, and great straggling of pigment particle distribution in the coating solution. Additionally, in regard to electrical properties, there are problems associated with cycle stability in sensitivity, residual potential and surface potential.